There is no magic number in prescribing the amount of water to consume throughout the day. The optimal amount varies according to body size, activity level, environmental factors, and diet. Drink just enough water throughout the day so your urine is clear. Drink extra water before, during and after physical activity or being exposed to higher temperatures. The goal is to drink before you get thirsty.
If physical activity last longer than 90 minutes (eg: sports competition), choose a sports drink with electrolytes. For even longer durations, consider a beverage with electrolytes, carbohydrates and protein or amino acids.
Dr Tim Noakes, MD, DSc, veteran of more than 70 marathons and ultra marathons and author of Lore of Running (2003) makes the following recommendations:
- Drink according to your thirst, your body will tell what it needs.
- Dehydration will not contribute in any way to any illnesses associated with prolonged exercise, including marathon, ultramarathons, and triathlons.
- There is no need to increase sodium above what your appetite dictates, even during exercise.
- If you are carbohydrate adapted, you should ingest some carbohydrates to optimize your performance during prolonged competitive events.
Understand that much of what you believe about your personal well-being is the result of targeted manipulations by industries whose principal focus is their commercial fitness and not necessarily your health or safety.
In addition, Noakes (1988) demonstrated that exercise intensity, not the level of dehydration, is the most important factor determining hyperthermia. His studies demonstrate that during prolonged exercise in mild environmental conditions, a fluid intake of 0.5 l.h-1 will prevent significant dehydration in the majority of athletes.
Noakes T (2012). Waterlogged, The Serious Problem of Overhydration in Endurance Sports.
Noakes TD, Adams BA, Myburgh KH, Greeff C, Lotz T, Nathan M. (1988). The danger of an inadequate water intake during prolonged exercise. A novel concept re-visited. Eur J Appl Physiol Occup Physiol. 57(2):210-9.
Also see Dr Noakes, video presentation Challenging Beliefs, where he talks more about water requirements during exercise.
Hyponatremia, or water intoxication is a potentially fatal condition caused by consuming too much water. Excessive water consumption causes low sodium levels in the blood. Symptoms may include nausea, muscle cramps, coma, and even death. A runner died from the condition following the 2002 Boston Marathon. Another runner was in a comma for 4 days following a marathon in Jamaica.
Effects of Dehydration
Dehydration of 3% of body weight can cause a 10% strength loss and 8% speed loss.
Cold water is absorbed faster in the body; sugar slows absorption.
Effect on Metabolic Rate
Drinking 500 ml of water increased metabolic rate by 30% in healthy, normal-weight subjects. The increase of metabolic rate occurred within 10 min and reached a maximum after 30-40 min. About 40% of the thermogenic effect originated from warming the water from 22 to 37 C. The total thermogenic response was about 24 Calories. It was estimated that drinking 2 liters of water per day would augment energy expenditure by approximately 96 Calories.
Boschmann M, Steiniger J, Hille U, Tank J, Adams F, Sharma AM, Klaus S, Luft FC, Jordan J. (2003). Water-induced thermogenesis. J Clin Endocrinol Metab. 88(12):6015-9.
Also see Foods that May Aid in Fat Loss.